It sounds like a developmental milestone! He's starting to be more aware of things around him and has interests other than sleeping. Go ahead and take him into your bed if it seems to help him (often it can be soothing, plus it makes any nighttime feedings easeir). Just make sure to get rid of any extra pillows, blankets, fluffy stuff, etc. And DON'T do this if either of you have a sleep disorder or take medication that makes you a heavy sleeper (for his safety).
I would also start introducing solids. Start with rice cereal mixed with breastmilk. I always fed the cereal in the morning, but you can do it at any time during the day. It might help him sleep a bit better.
At grandma's, check out where he sleeps. Is it similar to his room at home? Maybe he needs it to be darker, or maybe it's too noisy, or too quiet. Maybe it just feels different. See if you can adjust the environment there. Of course, it could be that he is more interested in other things, but he still needs his naps.
Last thing - if he is uncomfortable or squirms a lot when you put him down, he could have gas or acid reflux. You may want to mention it to the doc at his 6 month appointment, she may have some suggestions to help there